The present invention is directed to a soft, strong in use, bulky single ply tissue paper having low sidedness and processes for the manufacture of such tissues.
Through air drying has become the technology of preference for making tissue for many manufacturers who build new tissue machines as, on balance, through air drying (xe2x80x9cTADxe2x80x9d) offers many economic benefits as compared to the older techniques of conventional wet-pressing (xe2x80x9cCWPxe2x80x9d). With through air drying, it is possible to produce a single ply tissue with good initial softness and bulk as it leaves the tissue machine.
In the older wet pressing method, to produce a premium quality tissue, it has normally been preferred to combine two plies by embossing them together. In this way, the rougher air-side surfaces of each ply may be joined to each other and thereby concealed within the sheet. However, producing two-ply products, even on state of the art CWP machines, lowers paper machine productivity by about 20% as compared to a one-ply product. In addition, there may be a substantial cost penalty involved in the production of two-ply products because the parent rolls of each ply are not always of the same length, and a break in either of the single plies forces the operation to be shut down until it can be remedied. Also, it is not normally economic to convert older CWP tissue machines to TAD. But even though through air drying has often been preferred for new machines, conventional wet-pressing is not without its advantages as well. Water may normally be removed from a cellulosic web at lower energy cost by mechanical means such as by overall compaction than by drying using hot air.
What has been needed in the art is a method of making a premium quality single ply tissue using conventional wet pressing having a high bulk and excellent softness and absorbency attributes. In this way advantages can be taken of older CWP machines that can be used to produce high quality single ply tissue at a cost which is far lower than that associated with producing two-ply tissue.
Among the more significant barriers to production of a single ply CWP tissue have been the generally low softness and thickness and the extreme sidedness of single-ply webs. A tissue product""s softness can be increased by lowering its strength, as it is known that softness and strength are inversely related. However, a product having very low strength will present difficulties in manufacturing and will be rejected by consumers as it will not hold up in use. Use of premium, low coarseness fibers, such as eucalyptus, and stratification of the furnish so that the premium softness fibers are on the outer layers of the tissue is another way of addressing the low softness of CWP products; however this solution is expensive to apply, both in terms of equipment and ongoing fiber costs. In any case, neither of these schemes addresses the problem of low thickness. TAD processes employing fiber stratification can produce a nice, soft, bulky sheet having adequate strength and good similarity of the surface texture on the front of the sheet as compared to the back. Having the same texture on front and back is considered to be quite desirable in these products or, more precisely, having differing texture is generally considered quite undesirable. Because of the deficiencies mentioned above, many single-ply CWP products currently found in the marketplace are typically low-end products. These products often are considered deficient in thickness, softness, and absorbency, and they exhibit excessive two sidedness. Accordingly, these products have had rather low consumer acceptance and are typically used in xe2x80x9caway from homexe2x80x9d applications in which the person buying the tissue is not the user.
We have found that we can produce soft, high basis weight, high strength CWP tissues with low sidedness by the judicious combination of several techniques as described herein. Basically, these techniques fall into four categories: (i) providing a web having a basis weight of at least 15 pounds for each 3,000 square foot ream; (ii) adding to the web a controlled amount of a temporary wet strength agent and softener/debonder; (iii) low angle, high percent crepe, high adhesion creping to give the product low stiffness and a high stretch; and (iv) embossing the tissue between malted emboss rolls, each of which has both male and female elements. By various combinations of these techniques as described, taught, and exemplified herein, it is possible to control the required degrees of softness, strength, absorbency and sidedness for the desired end use.
Paper is generally manufactured by suspending cellulosic fiber of appropriate geometric dimensions in an aqueous medium and then removing most of the liquid. The paper derives some if its structural integrity from the mechanical arrangement of the cellulosic fibers in the web, but most, by far, of the paper""s strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this inter-fiber bonding, while necessary to the utility of the product, can result in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to Yankee drum thermal drying means with an adhesive/release agent combination and then scraping the web off the Yankee by means of a creping blade. Creping, by breaking a significant number of inter-fiber bonds increases the perceived softness of the resulting bathroom tissue product.
Another method of increasing a web""s softness is through the addition of chemical softening and debonding agents. Compounds such as quaternary amines that function as debonding agents are often incorporated into the paper web. These A cationic quaternary amines can be added to the initial fibrous slurry from which the paper web is subsequently made. Alternatively, the chemical debonding agent may be sprayed onto the cellulosic web after it is formed but before it is dried.
As was mentioned above, one-ply bathroom tissue generally suffers from the problem of low thickness, lack of softness, and also xe2x80x9csidedness.xe2x80x9d Sidedness is introduced into the sheet during the manufacturing process. The side of the sheet that was adhered to the Yankee and creped off, i.e., the Yankee side, is generally softer than the xe2x80x9cairxe2x80x9d side of the sheet. This two-sidedness is seen both in sheets that have been pressed to remove water and in unpressed sheets that have been subjected to vacuum and hot air (through-drying) prior to being adhered to the crepe dryer. The sidedness is present even after treatment with a softener. A premium one-ply tissue should not only have a high overall softness level, but should also exhibit softness of each side approaching the softness of the other.
The most pertinent prior art patents will be discussed but, in our view, none of them can be fairly said to apply to a one-ply tissue of this invention which exhibits high thickness, soft, strong and low sidedness attributes. U.S. Pat. No. 4,447,294, issued to Osborne, Ill., relates to towels and facial tissues and discloses a process for making a towel or facial tissue product having high wet strength and low dry strength. This reference requires that the wet strength agent be at least partially cured and that a debonding agent be applied to the already-dried web, which further distinguishes that reference from the present invention. Phan et al., in U.S. Pat. No. 5,262,007 discloses towels, napkins, and tissue papers containing biodegradable softening compound, a temporary wet strength resin, and a wetting agent. The Phan reference requires the use of a wetting agent, presumably to restore the absorbency lost by use of the softening agent The present invention is unrelated to the Phan reference and does not require use of a wetting agent to achieved a one-ply bathroom tissue having high absorbency. In U.S. Pat. No. 5,164,045, Awofeso et al. disclose a soft, high bulk tissue. However, production of this product requires stratified foam forming and a furnish that contains a substantial amount of anfractuous and mechanical bulking fibers, none of which are necessary to the present invention. U.S. Pat. No. 5,695,607 discloses a low sidedness product, but the tissue does not have the high thickness and temporary wet strength agent of the present invention. U.S. patent application Ser. No. **(case 1930) does not disclose mated embossing and the resulting product does not have as high a cross direction stretch or cross direction tensile energy absorbed for a given base sheet cross direction stretch and tensile energy absorbed. In addition, production of this product requires such strategies as fiber and/or chemical stratification that have been found unnecessary to produce the product of the present invention. Dunning et al., U.S. Pat. No. 4,166,001, discloses a double creped three-layered product having a weak middle layer. The Dunning product does not suggest the novel one-ply premium softness soft tissue of this invention and does not contain a temporary wet strength agent. The foregoing prior art references do not disclose or suggest a high-softness, strong one-ply tissue having low sidedness and having a total tensile strength of no more than 75 grams per three inches per pound per ream basis weight, A cross-machine direction stretch of at least 5.0 percent wherein the ratio of embossed product stretch to that of the base sheet is at least about 1.4, a cross direction wet tensile strength of at least 2.7 grams per three inches per pound per ream of basis weight, a tensile stiffness of less than about 1.1 grams per inch per percent strain per pound per ream basis weight, a GM friction deviation of no more than 0.225 and a sidedness parameter less than 0.275 usually in the range of about 0.180 to about 0.250.
The novel premium quality high-softness, single-ply tissue having a very low xe2x80x9csidednessxe2x80x9d along with excellent softness, coupled with strength is advantageously obtained by using a combination of four processing steps.
Suitably, the premium softness, strong, low sidedness bathroom tissue has been prepared by utilizing techniques falling into four categories: (i) providing a web having basis weight of at least 15 pounds for each 3,000 square foot ream; (ii) adding to the web or to the furnish controlled amounts of a temporary wet strength agent and a softener/debonder; (iii) low angle, high adhesion creping using suitable high strength nitrogen containing organic adhesives and a crepe angle of less than 85 degrees, the relative speeds of the Yankee dryer and a reel being controlled to produce a product MD stretch of at least 15%; and (iv) embossing the tissue between mated emboss rolls, each of which has both male and female elements. The furnish may include a mixture of softwood, hardwood, and recycled fiber. The premium softness and strong single-ply tissue having low sidedness may be suitably obtained from a homogenous former or from two-layer, three-layer, or multi-layer stratified formers.
Further advantages of the invention will be set forth in part in the description which follows. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing advantages and in accordance with the purpose of the invention as embodied and broadly described herein, there is disclosed:
A method of making an absorbent high-softness, high-basis weight, single-ply tissue comprising:
(a) providing a fibrous pulp of papermaking fibers;
(b) forming a nascent web from said pulp, wherein said web has a basis weight of at least about 15 lbs./3,000 sq. ft. ream;
(c) including in said web at least about 3 lbs./ton of a temporary wet strength agent and up to 10 lbs./ton of a nitrogen containing softener; optionally a cationic nitrogen containing softener;
(d) dewatering said web;
(e) adhering said web to a Yankee dryer;
(f) creping said web from said Yankee dryer using a creping angle of less than 85 degrees, wherein the relative speeds between said Yankee dryer and the take-up reel is controlled to produce a final product MD stretch of at least about 15%;
(g) optionally calendering said web;
(h) embossing said web between mated emboss rolls, each of which contains both male and female elements;
(i) forming a single-ply web wherein steps (a)-(f) and (h) and optionally step (g) are controlled to result in a single-ply tissue product having a total tensile strength of no more than 75 grams per three inches per pound per ream basis weight, a cross direction wet tensile strength of at least 2.7 grams per three inches per pound per ream of basis weight, a tensile stiffness of no more than about 1.1 grams per inch per percent strain per pound per ream basis weight, a ratio of product cross direction stretch to base sheet cross direction stretch of at least about 1.4, a GM friction deviation of no more than 0.225 and a sidedness parameter less than 0.275 usually in the range of about 0.180 to about 0.250.
There is also disclosed a single-ply tissue produced by a wet pressing technique, having a total tensile strength of no more than 75 grams per three inches per pound per ream basis weight, a cross direction wet tensile strength of at least 2.7 grams per three inches per pound per ream of basis weight, a tensile stiffness of no more than about 1.1 grams per inch per percent strain per pound per ream basis weight, a ratio of product cross direction stretch to base sheet cross direction stretch of at least about 1.4, a GM friction deviation of no more than 0.225 and a sidedness parameter less than 0.275 usually in the range of about 0.180 to about 0.250.